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Geologische Rundschau

, Volume 70, Issue 3, pp 972–1000 | Cite as

The Okahandja Lineament and its significance for Damaran tectonics in Namibia

  • Kevin N. Downing
  • M. P. Coward
Aufsätze

Abstract

The Okahandja Lineament is considered to represent the southern margin of a magmatic arc produced by the northerly subduction of the Damran Ocean between 750 and 520 Ma ago. Sediments of the Orogen and granitoid intrusives produced by subduction and crustal melting, underwent shear deformation between 675 and 575 Ma in a low angle zone of sinistral sense shear.

The Okahandja Lineament represents a zone of differential movement between the Central and Southern Zones of the Orogen during this simple shear deformation. Both overriding and overriden plates were deformed at about 550 and 520 Ma. Production of open S.E. verging structures on the northern plate was accompanied by more intense thrust and fold nappe production on the southern margin of the Damaran Orogen. During this last deformation event, the Central magmatic Zone was downfolded under the southern zone. This fold is the present expression of the Okahandja Lineament. Two tectonic models are proposed for the structures identified within the Central and Southern Zones of the Orogen, combined with evidence from the northern coastal arm of the Orogen.

Keywords

Subduction Orogen Southern Margin Southern Zone Sense Shear 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Zusammenfassung

Das Okahandja-Lineament wird als der Südrand eines „magmatischen Bogens“ (Wurzelzone eines Inselbogens) verstanden, der auf die nordwÄrts gerichtete Subduktion des Damara Ozeans von 750 bis 520 Ma zurückgeführt wird. Die Sedimente des Orogens und die granitoiden Intrusiva, die durch Subduktion und daraus folgender Aufschmelzung der Kruste entstanden, erlitten mehrfache linksgerichtete Scherdeformation entlang einer flachliegenden Zone zwischen 675 und 575 Ma.

Das Okahandja-Lineament stellt eine Zone von Teilbewegungen dar für die einfachen Scherdeformationen zwischen der zentralen und der südlichen Zone des Orogens. Die hangende und die liegende Platte wurden zwischen ca. 550 und 520 Ma deformiert. In der nördlichen Platte wurden offene SE-vergente Strukturen gebildet, wÄhrend gleichzeitig am südlichen Rand des Damara-Orogens durch die grö\ere Beanspruchung Falten-Decken entstanden. Dabei wurde die zentrale magmatische Zone unter die südliche Zone gebogen. Diese Falte ist das heutige Erscheinungsbild des Okahandja Lineaments.

Zwei tektonische Modelle werden dargelegt, um die erkannten Strukturen in der zentralen und südlichen Zone des Orogens unter Verwendung der Evidenz vom nördlichen Küstenarm des Orogens zu erklÄren.

Résumé

Le linéament de Okahandja est considéré comme la marge bordant au sud un «arc magmatique» résultant de la subduction vers le nord de l'Océan damaranique durant l'intervalle 750-520 M. a. - Les sédiments de l'orogène et les intrusions granitoÏdes produites par la subduction et par la fusion de la croûte subirent, entre 675 et 575 M. a., de nobreuses déformations cisaillantes, sénestres, le long d'une zone faiblement inclinée.

Le linéament de Okahandja représente, pour les déformations cisaillantes simples une zone de déplacements partiels entre la zone centrale et la zone méridionale de l'orogène. Les deux plaques, celle du toit et celle du mur, de ce chevauchement furent déformées entre approximativement 550 et 520 M. a. Ces structures ouvertes, déversées vers le nord, se formèrent dans la plaque septentrionale, pendant que des nappes en plis naissaient simultanément, sous l'influence d'une forte so sollicitation, dans la marge méridionale de l'orogène damaranique, ce qui entraÎna le ploiement de la zone magmatique centrale sous la zone méridionale. Ce pli est la forme sous laquelle apparaÎt aujourd'hui le linéament de Okahandja.

Deux modèles tectoniques sont proposés pour expliqeur les structures ainsi reconnues dans les zones centrale et méridionale de l'orogène, sur la base du fait d'un bras le long de la cÔte septentrionale de l'orogène.

кРАткОЕ сОДЕРжАНИЕ

лИНЕАМЕНт Okahandja РАссМАтР ИВАУт, кАк УжНыИ кРАИ „МАгМАтИЧЕскОИ ДУгИ” - кОРНИ ОстРОВНО И ДУгИ -, ВыжВАННОИ сУБД УкцИЕИ ОкЕАНА ДАМАРскОгО ВРЕМЕНИ 750-520 МИллИОНОВ л Ет тОМУ НАжАД, пРОстИР АУЩЕИсь НА сЕВЕР. жДЕсь ИМЕУтсь, кАк сЕДИМЕНт ы, тАк И гРАНИтОИДНыЕ п ОРОДы, пОьВИВшИЕсь В РЕжУль тАтЕ сУБДУкцИИ И пОслЕДУУ ЩЕгО РАсплАВлЕНИь кО Ры, — ВсЕ ОНИ пРЕтЕРпЕлИ МНОгОкРАтНыЕ ДЕФОРМ АцИИ скОльжЕНИь ВДОл ь НИжМЕННОстИ, И НАпРАВ лЕННыЕ ВлЕВО, МЕжДУ 675 И 575 МИллИО НАМИ лЕт тОМУ НАжАД.

лИНЕАМЕНт Okahandja пРЕДстАВ льЕт сОБОИ жОНы УМЕРЕ ННОгО ДВИжЕНИь, пРИЧЕМ МЕжДУ цЕНтРАльНОИ И У жНОИ жОНАМИ ОРОгЕНА И МЕлА МЕстО ДЕФОРМАцИь скОльжЕНИь. глыБА кРО ВлИ, кАк глыБА пОДОшВы пРЕтЕРпЕлИ ДЕФОРМАцИУ МЕжДУ 550 И 520 МИллИОНОВ лЕ т тОМУ НАжАД. НА сЕВЕРН ОИ глыБЕ ОБРАжОВАлАсь ОткРытАь стРУктУРА О пРОкИНУтАь НА УгОВОс тОк, В тО ВРЕМь, кАк В РЕжУльтАт Е НАпРьжЕНИь ВОжНИклИ склАДкИ пОкРОВА. пРИ Ё тОМ цЕНтРАльНАь МАгМАтИ ЧЕскАь жОНА пОД УжНОИ ЧАстьУ ОкАжАлАсь сМьтОИ. РАс пОлОжЕНИЕ ЁтИх склАДОк ОтРАжАЕт сЕгОДНьшНЕ Е РАспОлОжЕНИЕ лИНЕА МЕНтА.

Дль ОБЩьсНЕНИь ВОжНИ кНОВЕНИь ЁтИх стРУкт УРНых ЁлЕМЕНтОВ В цЕНтРАль НОИ И УжНОИ жОНАх ОРОгЕНА пРЕДлАгАУт ДВЕ тАктО НИЧЕскИЕ МОДЕлИ, В кОтОРых УЧтЕНы И стРУктУРНыЕ ЁлЕМЕНты сЕВЕРНОгО Б ЕРЕгОВОгО кРАь ОРОгЕНА.

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References

  1. Arendt, H., Hunziker, J. C. andWeber, K.: Age and degree of metamorphism and time of nappe emplacement along the southern margin of the Damara Orogen/Namibia (South West Africa). Geol. Rdsch. 67, 2, 719–742, 1978.Google Scholar
  2. Barnes, J. F. H. andDowning, K. N.: Origin of domes in the Central Damaran belt, Namibia. Revue de Geol. Dynam. et Geog. Physique, 21, 383–386, 1979.Google Scholar
  3. Bickle, M. J. andCoward, M. P.: A major thrust in the southern Damaran belt, Namibia. 20th ann. Rep. res. Inst. afr. Geol., Univ. Leeds, 8–17, 1977.Google Scholar
  4. Bickle, M. J., Hawkesworth, C. J., England, D. C. andAthey, D. R.: A preliminary thermal model for regional metamorphism in the Eastern Alps. Earth Plan. Sci. Letters., 26, 13–28, 1975.Google Scholar
  5. Blaxland, A., Gohn, E., Haack, U. andHoffer, E.: Rb/Sr ages of late tectonic granites in the Damara Orogen, South West Africa/Namibia. N. Jb. Minor. Min. 11, 498–508, Stuttgart, 1979.Google Scholar
  6. Bouillier, A. M., Davison, I., Bertrand, J. andCoward, M. P.: L'unite granulitique des Iforas: une nappe de socle d'age pan-Africain precoce. Bull. Soc. geol. France, XX, 6, 877–882, 1978.Google Scholar
  7. Burger, A. J., Clifford, T. N. andMiller, R. McG.: Zircon U-Pb ages of the Franzfontain granitic suite, northern South West Africa. Precamb. Res., 3, 415–431, 1976.Google Scholar
  8. Burger, A. J., von Knorring, O. andClifford, T. N.: Mineralogical and radiometric studies of monazite and sphene occurrences in the Namib Desert, South West Africa. Min. Mag., 35, 519–528, 1965.Google Scholar
  9. Clauer, N. andKröner, A.: Sr and Ar Isotopic homogenization of pelitic sediments during low-grade regional metamorphism: The Pan-African upper Damaran sequence of northern Namibia (South West Africa). Earth Plan. Sci. Letts., 43, 117–131, 1979.Google Scholar
  10. Cobbold, P. R. andQuinquis, H.: Development of sheath folds in shear regimes. J. Struct. Geol., 119–126, 1980.Google Scholar
  11. Coward, M. P.: The Junction between Pan African mobile belts in Namibia: its structural History. Tectonophysics 76, 59–74, 1981.Google Scholar
  12. Coward, M. P., Kim, J. H. andParke, J.: A correlation of Lewisian structures and their displacement across the lower thrusts of the Moine thrust zone, NW Scotland. Proc. Geol. Ass., 91, 327–337, 1980.Google Scholar
  13. Davies, C. T., andCoward, M. P.: The structural evolution of the Gariep Arc in southern Namibia (S. W. Africa). PRE-CAMBRIAN RESEARCH (in press).Google Scholar
  14. Downing, K. N.: The stratigraphy and palaeoenvironment of the Damaran Supergroup in the Okahandja Lineament area. South Af. Geol. Surv. Spec. publ. on the Damara Orogen, in press.Google Scholar
  15. Esquevin, J. andMenendez, R.: Age Rb/Sr des granites du permes Diana (Omakuara et Sandfiats, S.W.A.). Unpubl. S.N.P.A. report for Aquitaine S.W.A., Mineral Exploration 1974.Google Scholar
  16. Fleitont, L. andFroidevaux, C.: Thermal and mechanical evolution of shear zones. J. Struct. Geol., 2, 159–164, 1980.Google Scholar
  17. Gevers, T. W.: An ancient tillite in South West Africa. Trans. Geol. Soc. S. Afr., 34, 1–17, 1931.Google Scholar
  18. —: Geology along the northwestern margin of the Khomas Highlands between Otjimbinque-Karibib and Okahandja, South West Africa. Trans. geol. soc. S. Afr., 66, 199–258, 1963.Google Scholar
  19. Grey, D. R.: Morphologic classification of crenulation cleavage. Journ. Geol., 85, 229–235, 1977.Google Scholar
  20. Haack, U., Gohn, E. andKlein, J. A.: Rb/Sr ages of granitic rocks along the middle reaches of the Omaruru River and the timing of Orogenic events in the Damaran belt, Namibia. Contrib. Min & Pet. 74, 349–360, 1980.Google Scholar
  21. HÄlbich, I. W.: Structure and tectonics along the southern margin of the Damaran mobile belt, South West Africa. Annal. Univ. Stellenboch. Ser. Al (Geology), 2, 149–247, 1977.Google Scholar
  22. Harland, W. B.: Tectonic transpression in Caledonian Spitsbergen. Geol. Mag., 108, 27–42, 1971.Google Scholar
  23. Hartnady, C. J.: The structural geology of the Naukluft nappe complex South West Africa and its relationship to the Damara Orogenic belt. Bull. Pre. C. Res. Unit Cape Town, 25, in press.Google Scholar
  24. -: Tectonic evolution of the southeastern part of the Hakos-Auas Mountain zone in the Damara Orogenic Belt. 14th & 15th Ann. Repts. Pre. C. Res. Unit Univ. Cape Town, 171–182, 1978 (b).Google Scholar
  25. -: The stratigraphy and structure of the Naukluft nappe complex. 14th & 15th Ann. Repts. Pre. C. Res. Unit, Univ. Cape Town, 163–170, 1978 (c).Google Scholar
  26. Hawkesworth, C. J., Kramers, J. D. andMiller, R. McG.: Old Model Nd Ages in Namibian Pan-African rocks, Nature 289, 278–282, 1981.Google Scholar
  27. Hsu, K. J.: Thin-skinned plate tectonics during neo-Alpine orogenesis. Am. J. Sci., 279, 353–366, 1979.Google Scholar
  28. Jacob, R. E.: Geology and Metamorphic petrology of part of the Damara Orogen along the lower Swakop River, South West Africa. Bull. Pre. C. Res. Unit, Univ. Cape Town, 17, 1974.Google Scholar
  29. Jacob, R. E., Kröner, A. andBurger, A. J.: Areal extent and first U-Pb age of the pre-Damaran Abbabis complex in the central Damara belt of South West Africa (Namibia). Geol. Rdsch. 67, 706–718, 1978.Google Scholar
  30. Kasch, K. W.: A continental collision model for the tectono-thermal evolution of the (Southern) Damara Belt. Ann. Rep. Pre. C. Res. Unit, Univ. Cape Town, 16, 101–107, 1979.Google Scholar
  31. Kennedy, W. Q.: The structural differentiation of Africa in the Pan-African (±500 m. y.) tectonic episode. 8th ann. Rep. res. Inst. afr. Geol., Univ. Leeds, 48–49, 1964.Google Scholar
  32. Kröner, A.: Chronology evolution of the Pan African Damara belt in Namibia, South West Africa. In Closs et al. (Eds.). Mobile Earth, final rept. of the Geodynamic project F R. G. Boldt. Verlag. Boppard, 221–224, 1980.Google Scholar
  33. Kröner, A. andHawkesworth, C. J.: Late Pan-African emplacement ages for Rössing alaskitic granite (Damara belt) and Rooi Lepel bostonite (Gariep belt) in Namibia and their significance for the timing of metamorphic events. 20th ann. Rep. res. Inst. afr. Geol., Univ. Leeds, 14–17, 1977.Google Scholar
  34. Kröner, A., Halper, M. andJacob, R. E.: Rb/Sr geochronology in favour of polymetamorphism in the Pan African Damara belt of Namibia (South West Africa). Geol. Rdsch. 67, 688–706, 1978.Google Scholar
  35. Le Roex, H. D.: A tillite in the Otavi Mountains, South West Africa. Trans. Geol. Soc. S. Afr., 44, 207–218, 1941.Google Scholar
  36. Malling, S.: Some aspects of the lithostratigraphy and tectono-metamorphic evolution in the Nauchas-Rehoboth area, S.W.A. (Namibia). 14th & 15th Ann. Repts. Pre. C. Res. Unit, Cape Town, 183–193, 1978.Google Scholar
  37. Marlow, A. G.: Geology and geochronology of mineralized and radioactive granites and alaskites, Namibia. South Afr. Geol. Surv. Spec. publ. on the Damara Orogen, in press.Google Scholar
  38. Martin, H.: The Precambrian geology of South West Africa and Namaqualand. Bull. Pre. C. Res. Unit, Univ. Cape Town, 4, 1–159, 1965.Google Scholar
  39. Martin, H. andPorada, H.: The intracratonic branch of the Damara Orogen in South West Africa. I, Discussion of geodynamic models. Precamb. Res., 5, 311–338, 1977.Google Scholar
  40. McElhinny, M. W. andMcWilliams, M. D.: Precambrian Geodynamics. A palaeomagnetic view. Tectonophysics, 40, 137–159, 1977.Google Scholar
  41. McIntyre, G. A., Brooks, C., Compston, W. andTurek, W.: The statistical assessment of Rb-Sr isochrons. J. Geophys. Res., 71, 5459–5468, 1966.Google Scholar
  42. Miller, R. McG.: The Okhandja Lineament. A fundamental tectonic boundary in the Damara Orogen of South West Africa/Namibia. Trans. geol. Soc. S. Afr., 82, 349–361, 1979.Google Scholar
  43. Porada, H.: The Damara-Ribeira orogen of the Pan-African-Brasillian cycle in Namibia (South West Africa) and Brazil as interpreted in terms of Continental Collision. Tectonophysics, 57, 237–265, 1979.Google Scholar
  44. Porada, H. andWittig, R.: Zur Tektonik des südlichen Damara-Belts. Sonderforschungsbericht 48 B (unpubl.), Univ. Göttingen, 51–70, 1975.Google Scholar
  45. Quinquis, H., Audren, C. L., Brun, J. P. andCobbold, P. R.: Intence progressive shear in Ile de Groix blueschists and compatibility with subduction or obduction. Nature, 273, 43–45, 1978.Google Scholar
  46. Ramsay, J. G.: Shear zone geometry: a review. J. Struct. Geol., 2, 83–100, 1980.Google Scholar
  47. Sawyer, E. W.: Damaran structural and metamorphic geology of an area southeast of Walvis Bay, South West Africa/Namibia. M. Sc. thesis (unpubl.), Univ. Cape Town, 1978.Google Scholar
  48. Smith, D. A. M.: The geology of the area around the Khan and Swakop Rivers in South West Africa. Mem. geol. Surv. S. Afr., 3 (S.W.A. Series), 113 pp., 1965.Google Scholar
  49. York, D.: Least-squares fitting of a straight line with correlation errors. Earth Plan. Sci. Lett., 5, 320–324, 1969.Google Scholar

Copyright information

© Ferdinand Enke Verlag Stuttgart 1981

Authors and Affiliations

  • Kevin N. Downing
    • 1
  • M. P. Coward
    • 1
  1. 1.Department of Earth SciencesThe UniversityLeedsGreat Britain

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